1. Field of the Invention
The invention relates to integrated circuit (IC) metrology and more particularly to a method and system for quality monitoring of photolithography processing.
2. Related Art
Growing demands for silicon wafers with large-scale integration necessitates submicron integrated circuit (IC) features with high precision and uniformity. As the features become smaller, it is increasingly critical to monitor the photolithographic process under which such semiconductor wafers are created.
In a typical photolithographic process, a series of masks are applied in a preset sequence to a silicon wafer, each mask is used to transfer its circuitry pattern onto a photosensitive layer (i.e., a photoresist layer) that is coated onto a layer (e.g., metal layer, etc.) formed on the silicon wafer.
FIG. 1 is a flow chart 100 illustrating conventional process steps for transferring a circuitry pattern from a mask to a silicon wafer. A silicon wafer is coated with photoresist in Step 110, and the wafer is then exposed to light emitted from a device such as a scanner or stepper in Step 120. The photoresist on the wafer is then developed to form structures on the photoresist layer in Step 130, and the developed wafer is inspected to ensure the measurements of critical dimension (CD) of the structures are within that set by a design rule. The design rule regulates features such as the minimum width of a line or the minimum spacing between two lines in order to ensure that the lines do not overlap or unintentionally interact. The wafer is then selectively etched according to the pattern imprinted on the photoresist layer in Step 140. Subsequent to etching, the silicon wafer is stripped of any remaining photoresist in Step 150, and the CD measurements are again inspected in a final inspect Step 160.
As feature sizes shrink, inspection and correction of surface features cross-sectional shape (“profile”), as well as CD measurements are crucial to higher yield and device performance.
Conventional methods employ a scanning electron microscope (SEM) known as a critical dimension scanning electron microscope (CD-SEM) to inspect and measure the profile and CD measurements on a wafer. However, CD-SEM is costly and must be employed in a vacuum environment.